As an important monomer in synthetic rubber and synthetic resin, butadiene is mainly used for synthesizing butadiene rubber, styrene butadiene rubber, nitrile rubber, ABS resin, etc. Besides, butadiene also serves as a feed stock in the preparation of coatings and in some organic chemical reactions.
Oxidative dehydrogenation of butene is currently a competitive process in producing butadiene, wherein butadiene and water of stable structures are obtained by the combination of oxygen and hydrogen in butene molecules in the presence of steam. Oxidative dehydrogenation of butene is substantially irreversible, with the main reaction equation as follows:2C4H8+O2→2C4H6+2H2O+127.9 kJ/mol
And the following reaction equations reflect the side reactions in oxidative dehydrogenation of butene:C4H8+4O2→4CO+4H2OC4H8+6O2→4CO2+4H2O
The main factors that would influence oxidative dehydrogenation of butene involve reaction temperatures, reaction pressures, the ratio of water to butene, the ratio of oxygen to butene, etc. In the reaction process, it is necessary to introduce a great deal of steam to facilitate protection of catalysts and control of the reaction temperature. Generally, the molar ratio of steam to the feed stock of butene reaches 8:1 to 16:1 or even higher. As to oxygen, since it participates in both the main reaction and side reactions, the adding amount thereof not only influences the conversion of butene, but also determines the degree to which the main and side reactions take place. Where the molar ration of oxygen to butene is too high, there would be more oxygen compounds and complete oxidation of butene as well, while too low a ratio of oxygen to butene would severely lower the conversion of butene. Furthermore, the adding amount of oxygen also concerns safety of the production. For example, the raw material formulation can by no means fall within the explosion limit. Moreover, inhomogeneous distribution of oxygen can also raise safety problems. Hence, feed stocks such as butene, air, and steam must be homogenously mixed after entering the oxidative dehydrogenation reactor. Otherwise, different amounts of steam at different catalyst bed layers in the reactor would cause carbon deposit rather fast. As can be concluded, the mixing homogeneity of the feed stocks is of essential importance to use of the overall oxidative dehydrogenation technology.
An axially fixed bed is now widely adopted in the preparation of butadiene by oxidative dehydrogenation of butene, as being recited, for example, in CN101367702. Although the axially fixed bed is of simple structures, the height of a catalyst bed layer thereof is restricted in order to satisfy the requirement of lowering the pressure drop. Furthermore, where an axially fixed bed is used, the production scale of the apparatus for oxidative dehydrogenation of butene would usually fall within the range from 5,000 to 15,000 tons per year. Nevertheless, as butadiene is increasingly required and the oxidative dehydrogenation technology is progressed, the production scale of an apparatus for preparing butadiene reaches more than 100,000 tons per year. Hence, four or even more reaction lines, such as 8 axial reactors divided into 4 groups, are required in the apparatus comprising such axially fixed bed reactors, which results in complex operations, and large investment and floor areas.
CN2626604Y discloses a radial reactor with a fluid substantially distributed in a homogeneous manner. However, this radial reactor fails to achieve homogeneous mixing of fluids entering a passage, and is of complicated structures and large pressure drop. CN102675027A, for example, discloses a process for preparing butadiene by oxidative dehydrogenation of butene using a radially fixed bed, wherein the apparatus thereof is also of complicated structures and large pressure drop, and is silent on a very important issue, i.e., homogeneous mixing of feed stock gasses, whereby the industrial use of oxidative dehydrogenation of butene is significantly limited.